1. Field
The present disclosure relates to an improved technique relating to a power steering apparatus for a vehicle and a method of producing the same. The power steering apparatus includes a torque sensor for detecting the amount and direction of torque acting on the column shaft (steering shaft) connected to the steering wheel.
2. Description of the Related Art
Examples of the power steering apparatus for a vehicle include an electric power steering apparatus. The electric power steering apparatus detects a steering torque generated on the column shaft (steering shaft), which is connected to the steering wheel, at a torque sensor, and generates an auxiliary torque corresponding to the steering torque, which is combined with the steering torque, at an electric motor. Thus, the electric power steering apparatus reduces the workload of a driver.
To simplify the configuration of the electric power steering apparatus, a technology using a magnetostrictive torque sensor, for example, has been developed in recent years. Particularly, an electric power steering apparatus configured to transmit auxiliary torque to the column shaft is called a column-assist electric power steering apparatus. Such a column-assist electric power steering apparatus is disclosed in Japanese Unexamined Patent Application Publication No. 2013-233879, for example.
The technology described in Japanese Unexamined Patent Application Publication No. 2013-233879 includes a steering wheel, a column shaft connected to the steering wheel, a column pipe rotatably supporting the column shaft through a plurality of bearings, and a torque sensor configured to detect a steering torque generated on the column shaft by using magnetism.
The torque sensor includes a magnetostrictive film disposed on an outer peripheral surface of the column shaft, an excitation coil, and a detection coil, which are disposed around the magnetostrictive film. The magnetostrictive film, which is a magnetically variable portion (a magnetostrictive portion), has, due to an inverse magnetostrictive effect, magnetostriction which varies with the steering torque generated on the column shaft. The detection coil, which is a magnetic change detector, detects a change in the magnetism of the magnetostrictive film.
The detection coil is housed inside a lower end portion of a cylindrical sensor housing. An upper end portion of the sensor housing is connected to a lower end portion of a column pipe. In other words, the detection coil is not surrounded by the column pipe. The column shaft is supported at an upper end portion of the column pipe by upper bearings and supported at the lower end portion of the sensor housing by lower bearings.
The torque sensor disclosed in Japanese Unexamined Patent Application Publication No. 2013-233879 is likely to be affected by outside magnetic noise. To improve and stabilize the detection accuracy of the torque sensor, the magnetic noise should be reduced as much as possible so as not to affect the torque sensor. To reduce the effect of the magnetic noise, the torque sensor may be magnetically shielded by a magnetic shield.
However, magnetically shielding the torque sensor by using the magnetic shield increases the number of components. The increased number of components makes the configuration of the torque sensor complex and increases the cost and the weight of the torque sensor. Thus, the torque sensor requires further improvement.